How to check if an image has transparency using GD?

Question

How do I check if an image has transparent pixels with php\'s GD library?

Answer 1

This is how I detect 8-32 bit transparency. It only work with PNG's.

function detect_transparency($file){

    if(!@getimagesize($file)) return false;

    if(ord(file_get_contents($file, false, null, 25, 1)) & 4) return true;

    $content = file_get_contents($file);
    if(stripos($content,'PLTE') !== false && stripos($content, 'tRNS') !== false) return true;

    return false;
}